1. Field of the Invention
The present invention relates to a high-voltage electric double layer capacitor (EDLC), and more particularly, to an EDLC in which a surge voltage and an operating voltage are enhanced by improving the structure of a unit cell.
2. Description of the Related Art
An EDLC is an energy storage medium in which two electrodes of the anode and the cathode are arranged opposite to each other with insulating paper interposed therebetween so that a pair of charge layers (electric double layers) having different signs can be generated on facing surfaces of the electrodes. The EDLC can be continuously charged and discharged. This EDLC is mainly used as an auxiliary power supply, an IC backup power supply or the like for a variety of electrical and electronic devices. Recently, the EDLC has been widely used for a toy, an industrial power supply, an uninterrupted power supply (UPS), solar energy storage, HEV/EV sub power, and the like.
The EDLC is generally fabricated by accommodating a unit cell in a casing and then filling the casing with an electrolyte. At this time, the unit cell is constructed by alternately laminating electrodes and sheets of insulating paper and forming them into a roll. In order to establish a proper voltage and capacitance required for an EDLC, two or more unit cells are connected in series and in parallel to form the EDLC. This will be described in detail below with reference to FIGS. 1 to 4. FIG. 1 is a sectional view showing a laminate structure of a unit cell according to a prior art. FIG. 2 is a perspective view illustrating a state where the unit cell shown in FIG. 1 is wound. FIG. 3 is a perspective view illustrating a state where the unit cell shown in FIG. 1 is compressed. FIG. 4 is a sectional schematic view showing the distribution and polarized state of charges in the unit cell shown in FIG. 1.
Referring to FIGS. 1 and 2, the conventional unit cell includes two sheets of insulating paper 1a and 1b and two electrodes 2a and 2b. In this case, the electrodes 2a and 2b and the sheets of insulating paper 1a and 1b are alternately laminated to insulate the two electrodes 2a and 2b from each other, and the laminate is then rolled. That is, according to the prior art, the unit cell is constructed by sequentially laminating a first insulating paper layer 1A with a sheet of insulating paper 1a, a first electrode layer 2A with one electrode 2a, a second insulating paper layer 1B with a sheet of insulating paper 1b, and a second electrode layer 2B with one electrode 2b, as shown in FIG. 1.
The laminate thus constructed is rolled as shown in FIG. 2 to form the unit cell. At this time, the electrodes 2a and 2b have positive polarity (+) or negative polarity (−) determined according to the sign of external electricity applied thereto. Terminals 2a′ and 2b′ to which external electricity is applied are drawn from the electrodes 2a and 2b. 
FIG. 4 shows that the first electrode layer 2A has negative polarity (−) and the second electrode layer 2B has positive polarity (+).
The unit cell rolled as such may be generally contained directly in a cylindrical case, or in a polygonal or bag-shaped (usually, pouch-shaped) case if it is compressed as shown in FIG. 3.
As described above, the conventional unit cell is constructed to have the two electrodes 2a and 2b. That is, the unit cell is constructed to have the first electrode layer 2A and the second electrode layer 2B, wherein each of the electrode layers comprises one electrode and the electrodes of the unit cell comprise the two electrodes 2a and 2b as the anode (+) and the cathode (−). Further, the unit cell has charge distribution as shown in FIG. 4. In the unit cell, positive (+) charges and negative (−) charges are polarized in the first electrode layer 2A and the second insulating paper layer 1B, and in the second insulating paper layer 1B and the second electrode layer 2B. Accordingly, two charge layers (electric double layers) are formed in a single unit cell.
In the conventional unit cell constructed as above, however, a surge voltage is low, i.e., less than 3.0V, and an operating voltage is also low, i.e., 2.3V to 2.7V. Therefore, there is a problem in that two or more unit cells have to be connected in series in order to establish an operating voltage applicable to electronic products.
The EDLC has the area of facing surfaces (specific surface area) increased with the use of activated carbon electrodes, and capacitance improved with the use of an electrolyte, as compared with existing other capacitors. As described above, the EDLC has the two sheets of insulating paper 1a and 1b for separating the two opposite electrodes 2a and 2b. In particular, a cylindrical unit cell to which the roll shape shown in FIG. 2 is applied is obtained by rolling the laminate such that the electrodes face each other in both the front and rear of them to improve capacitance. This can be considered to increase the area of facing surfaces. This can increase capacitance as expressed by the following equation, but has nothing to do with increase in voltage:C=ε·S/dwhere C: capacitance, ε: dielectric constant, S: cross section of electrode, and d: distance between opposite electrodes.
Furthermore, the EDLC, which is an energy storage medium enabling instant charge/discharge, has a superior output characteristic to that of a battery but has a low voltage per unit cell since its voltage gradually drops simultaneously with discharge. Therefore, the EDLC has an energy storage density smaller than that of the battery. Accordingly, the EDLC is generally used for an auxiliary power supply for the output of a battery, and an auxiliary power supply for other electrical and electronic devices.
Most of electronic products including ICs and backup power supply products have an operating voltage of 1.8V or more, preferably a wide voltage range of 3V to 48V (in case of electric cars). Accordingly, in order for the EDLC to be used for these products, two or more unit cells are serially connected to increase the operating voltage to at least 5V. In order for the EDLC to be used for industrial apparatuses, electric cars and UPSs, 10 to 100 unit cells are connected in series/parallel to a range of 10V to 48V.
However, in the case where two or more unit cells (each 3.0V or less) are serially connected to increase the operating voltage of a capacitor, there is another problem in that a balance problem between the unit cells, which inevitably occurs, should be solved. Specifically, there is a need for a voltage balance protection circuit such as a resistor, a diode and another IC so that the overall operating voltage of the capacitor is not concentrated on one unit cell, in consideration of the capacitance of the unit cell, equivalent serial resistance (ESR), a leakage current and the like of a unit cell. At this time, a more important fact is that the aforementioned packaging has to be formed such that the overall operating voltage is significantly lower than the operating voltage of each unit cell by adding about one unit cell in order to give a margin for the overall operating voltage. That is, for the purpose of stable operation, a module should be fabricated to have a voltage of about 2.0V per cell.
Meanwhile, in an energy storage medium, a unique value is used to indicate the amount of energy that can be stored therein. In case of a battery, 1 AH (storage capacity capable of supporting the use of a current of 1 A for an hour) is used to indicate the amount of energy. In case of an EDLC, however, F (Farad) is used. Since the voltage of the EDLC varies simultaneously with discharge, F is used in accordance with the capacitance notation of a general condenser (capacitor).
Furthermore, an ultra-high capacitance EDLC has a capacitance that is one thousand to one million times larger than that of a general condenser with capacitance on the order of mF, uF or the like. However, a conventional EDLC has an operating voltage that is significantly lower than that of an existing battery or condenser, as described above.
The energy storage amount that is a piece of data related to energy storage can be considered as a good index useful for comparison of the amount of energy even in the EDLC, in the same manner as a battery and a condenser. The energy storage amount can be obtained by the following equation:Largest energy storage amount (J)=½ CV2 where C is capacitance per cell (F) and V is a voltage applicable to a cell.
From the above equation, it can be seen that the largest energy storage amount is proportional to capacitance but is proportional to the square of voltage. In order words, if the voltage increases twice for the same area, the largest energy storage amount increases four times. If the capacitance increases twice, however, the largest energy storage amount increases twice. Accordingly, it can be said that the best way to increase the value of the largest energy storage amount available in an EDLC is to increase the voltage.
As described above, however, in the prior art, unit cells are serially connected to increase an operating voltage. This method has a problem in that a high voltage is applied to any one of the unit cells since voltage balance among the unit cells is broken due to repeated cycles in the capacitance of a unit cell, a capacitance change rate, an ESR, a resistance changing rate, a leakage current, and a leakage current change rate. This causes an electrolyte to be dissolved (the electrolyte is dissolved when a voltage of 3.0V or more is applied thereto). It also results in increased internal resistance, lowered capacitance, and the like.
Therefore, it can be said that increase in a surge voltage and an operating voltage of an EDLC is the most important technical problem to be solved in the art. In this case, it is necessary to guarantee physical properties of the EDLC, such as internal resistance, capacitance, rapid charge/discharge, and instant high output.